Because the only color that works halfway well with them right now is blue, which is one of the darkest choices.
Neutral colors would fit them perfectly.
Before anyone says "but if I picked black I wouldn't have eyes", remember how the rest of the colors have the Knight's facial features a brighter color than the rest of their face- if you picked black, your eyes might be a dark grey. Crisis averted.
+1
Don't know why we don't have any of these already.